Implement speed up of images download

[benoit74]

In #367, we've implemented something to slow down image download.

The fact there is nothing to speed up when things seems to cool down is a kind of issue.

In this PR, we let scraper speed up a bit once many files have succeeded to download in a row.

[codecov]

Codecov Report

❌ Patch coverage is 21.05263% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 7.35%. Comparing base (f240a45) to head (fe3979c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sotoki/utils/imager.py	0.00%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #369      +/-   ##
========================================
+ Coverage   7.20%   7.35%   +0.15%     
========================================
  Files         24      24              
  Lines       2249    2256       +7     
  Branches     294     294              
========================================
+ Hits         162     166       +4     
- Misses      2086    2089       +3     
  Partials       1       1              

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